The aim of this work was to determine the antioxidant activities of a
range of phytoestrogenic isoflavones. The antioxidant activity in the
aqueous phase was determined by means of the ABTS(.+) total antioxidan
t activity assay. The results show that the order of reactivity in sca
venging the radical in the aqueous phase is genistein > daidzein = gen
istin approximate to biochanin A = daidzin > formononetin approximate
to ononin, the latter displaying no antioxidant activity. The importan
ce of the single 4'-hydroxyl group in the reactivity of the isoflavone
s, as scavengers of aqueous phase radicals, as well as the 5,7-dihydro
xy structure is demonstrated. Examination of their abilities to enhanc
e the resistance of low density lipoproteins to oxidation supports the
observation that genistein is the most potent antioxidant among this
family of compounds studied, both in the aqueous and in the lipophilic
phases.